2-40Figure 2-38.Fixed spark-gap modulator.The spark gap is actually triggered (ionized) by the combined action of the charging voltage acrossthe pulse-forming network and the trigger pulse. (Ionization was discussed in NEETS, Module 6,Introduction to Electronic Emission, Tubes and Power Supplies.) The air between the trigger pulseinjection point and ground is ionized by the trigger voltage. This, in turn, initiates the ionization of thecomplete gap by the charging voltage. This ionization allows conduction from the charged pulse-formingnetwork through pulse transformer T1. The output pulse is then applied to an oscillating device, such as amagnetron.Thyratron ModulatorThe hydrogen THYRATRON MODULATOR is an electronic switch which requires a positivetrigger of only 150 volts. The trigger potential must rise at the rate of 100 volts per microsecond to causethe modulator to conduct. In contrast to spark gap devices, the hydrogen thyratron (figure 2-39) operatesover a wide range of anode voltages and pulse-repetition rates. The grid has complete control over theinitiation of cathode emission for a wide range of voltages. The anode is completely shielded from thecathode by the grid. Thus, effective grid action results in very smooth firing over a wide range of anodevoltages and repetition frequencies. Unlike most other thyratrons, the positive grid-control characteristicensures stable operation. In addition, deionization time is reduced by using the hydrogen-filled tube.Figure 2-39.Typical thyratron gas-tube modulator.The hydrogen thyratron modulator provides improved timing because the synchronized trigger pulseis applied to the control grid of the thyratron (V2) and instantaneous firing is obtained. In addition, only